If $a + b = -1$ and $x + y + z = -10$, what is $6a - 9y - 9x + 6b - 9z$ ?
$= 6a + 6b - 9x - 9y - 9z$ $= (6) \cdot (a + b) + (-9) \cdot (x + y + z)$ $= (6) \cdot (-1) + (-9) \cdot (-10)$ $= -6 + 90$ $= 84$